The chemical mechanical planarization (CMP) process in a process for preparing semiconductors refers to a step in which a wafer is fixed to a head and in contact with the surface of a polishing pad mounted on a platen, and the wafer is then chemically treated by supplying a slurry while the platen and the head are relatively moved to thereby mechanically planarize the irregularities on the wafer surface.
A polishing pad is an essential member that plays an important role in such a CMP process. In general, a polishing pad is composed of a polyurethane-based resin and has grooves on its surface for a large flow of a slurry and pores for supporting a fine flow thereof.
The pores in a polishing pad may be formed by using a solid phase foaming agent having voids, a liquid phase foaming agent filled with a volatile liquid, an inert gas, a fiber, or the like, or by generating a gas by a chemical reaction.
The technique of forming pores using an inert gas or a volatile liquid phase foaming agent has the advantage that any material that may affect the CMP process is not discharged. However, it is difficult to precisely control the diameter of pores and the density of the pad since it is inevitable to deal with a gas phase, which is not conveniently controlled. It is particularly difficult to produce uniform pores of 50 μm or less. In addition, there is a problem that it is very difficult to control the diameter of pores and the density of the pad without changing the composition of the polyurethane matrix of the polishing pad.
As the solid phase foaming agent, microcapsules (i.e., thermally expanded microcapsules), whose size has been adjusted by a thermal expansion, are used. Since the thermally expanded microcapsules in a structure of already expanded micro-balloons have a uniform particle diameter, the diameter of pores can be uniformly controlled. For example, Korean Patent No. 1608901 discloses a polishing pad, which is formed using a solid phase foaming agent and comprises pores having a uniform diameter.